If users searched the help for how to change the default page size or page margins in Writer, they wouldnt get any useful results, so we need to create a new help page for this, as beginning level users arent aware of templates.
The new help file would a duplicate of the existing "Changing the Default Template" help file ( /source/text/swriter/guide/template_default.xhp ), but tweaked to mention opening the page dialog.
I'm in favor of making practical help with key instructions like this one proposed. I'm not user about what's proposed to duplicate something. Instruction should be linked to https://help.libreoffice.org/index.php?title=Common/Pag, maybe in "Related Topics".
Seth Chaiklin committed a patch related to this issue.
It has been pushed to "master":
Partially resolves: tdf#108007 / resolves: tdf#137401 changing page size and margins